North korea has raised the ante specifically on nuclear weapons approximate at the skruft secretary of state called the threat provocative and reckless. Abc's martha raddatz is the only american reporter to make her way to the bder with north korea today, to talk to the american general, literally on the front lines. Reporter: We stood just yards from north korea's red hot border. Its soldiers watching our every move in this buffer zone between north and south, with more than a million and a half land mines reinforcing the divide. General james thurman is commander of all u.S. Forces here. His job, to help protect america's ally, south korea, to prevent war, an increasingly difficult job with the alarming threats coming form the north's new young leader. The situation is volatile and it is dangerous. Reporter: What is your greatest fear right now? A miscalculation, an impulsive decision causes a kinetic provocation. There's 14,000 tubes of artillery just across this line, beyond that far mountain range over there. If they decided to, you know, resume hostilities, we got to be ready to go. Reporter: And the threat of hostilities is far greater than artillery rounds. The north koreans, so close to us today along this shared border that they peered through windows as we spoke, now vowing to expand their nuclear capabilities, already believed to have enough material for more to rebuild a nuclear reactor, blown up in 2008 to show the world it was committed to peace. And while they are not yet believed to have the ability to put a nuclear weapon on a missile that could reach the u.S., N is preparing for anything. Those are not empty threats? No, I don't think they are. We've got to take every threat seriously. Reporter: General thurman is still hoping he can diffuse this situation, but seeing those fortifications up close today, and feeling the tension of the soldiers, makes you realize how serious this could become.

This transcript has been automatically generated and may not be 100% accurate.

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